The activity of ornithine decarboxylase (ODC), the initial enzyme in polyamine biosynthetic pathway, fluctuates during the growth cycle of neuroblastoma and glioma cells. The activity of the enzyme is 1000 times higher in proliferating cells compared with stationary ones. This study indicates that cAMP mediates the induction of ODC. Growth of neural cells may thus be regulated by cAMP modulating cellular ODC and polyamine levels.